Active filter for power distribution system with selectable harmonic elimination

ABSTRACT

A power distribution system includes an ac power source; a power bus connected to the ac power source; a capacitor bank shunt-connected to the power bus; and an active filter shunt-connected to the power bus. The active filter includes current sensors, an inverter and an inverter control. Each current sensor senses current flowing through a corresponding capacitor of the capacitor bank. The inverter control, in response to the current sensors, controls the inverter to inject harmonic currents into the power bus.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to electrical power distributionsystems. More specifically, the present invention relates to a methodand apparatus for injecting harmonic currents into constant frequencyand variable frequency distribution systems.

[0002] Many aircraft include power distribution systems for supplying acpower to loads such as ac-to-dc converters, ac-to-ac converters,on-board electronics, and electromechanical/electrohydrostaticactuators. Many of these loads contain non-linear devices such as bridgerectifiers and inverters.

[0003] The non-linear devices can create harmonic currents on thesystem's power distribution line. The harmonic currents can disruptother loads connected to system's main power supply (e.g., electricalgenerators, inverters) and cause malfunction, and even failure, of theother loads.

[0004] It would be desirable to remove the harmonic currents withoutcutting into the power distribution line and measuring main distributioncurrent. In aircraft power distribution systems, the main distributioncurrent can be quite large. For example, a 115/208 volt, three-phase 150KVA generator has a rated current of approximately 450 amperes.

SUMMARY OF THE INVENTION

[0005] According to one aspect of the present invention, a powerdistribution system includes an ac power source; a power bus connectedto the ac power source; a capacitor bank shunt-connected to the powerbus; and an active filter shunt-connected to the power bus. The activefilter includes current sensors, an inverter and an inverter control.Each current sensor senses current flowing through a correspondingcapacitor of the capacitor bank. The inverter control, in response tothe current sensors, controls the inverter to inject harmonic currentsinto the power bus, thus supplying the harmonic current demands of thenon linear loads.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is an illustration of a power distribution system accordingto the present invention.

[0007]FIG. 2 is an illustration of an active filter for the powerdistribution system of FIG. 1.

[0008]FIG. 3 is an illustration of a control methodology for the activefilter.

[0009]FIG. 4 is an illustration of an overcurrent regulator for theactive filter.

[0010]FIG. 5 is an illustration of an overcurrent regulator thatoperates sequentially on control loops of the active filter.

[0011]FIGS. 6 and 7 are illustrations of two different voltage controlsfor a dc link capacitor of the active filter.

[0012]FIG. 8 is an illustration of a harmonic extractor.

DETAILED DESCRIPTION OF THE INVENTION

[0013] Reference is made to FIG. 1, which illustrates a single linediagram of a power distribution system 10 for an aircraft. Thedistribution system 10 includes an ac power source 12 and a power bus14. The power source 12 provides three-phase ac power to the bus 14 at afixed or variable fundamental frequency. The fundamental frequency isusually between 350 Hz and 700 Hz.

[0014] The ac power source 12 is not limited to any particular type. Asan example, the ac power source 12 may include a single high speedpermanent magnet generator, an inverter and an inverter control. Asanother example, the ac power source 12 may include wound fieldsynchronous generators that are controlled to produce a constant outputvoltage at variable frequency. Yet another example is a variable speedconstant frequency (VSCF) source that rectifies generator output voltageto dc, and then converts the dc power to ac power.

[0015] Multiple loads are connected to the power bus 14 to receive acpower. These loads are represented collectively by a single block 16.The loads 16 typically include non-linear devices that can give rise toharmonic currents on the power bus 14.

[0016] Rectifiers and inverters, which constitute a portion of thenon-linear loads, can be modeled as current sources that inject harmoniccurrents into the power bus 14. The harmonic currents have frequenciesthat are a multiple of the fundamental frequency of the ac powergenerated by the power source 12.

[0017] An inductor 18 represents the inductance in the powerdistribution system 10. The inductor 18 also represents source impedanceassociated with the power sources connected to the power bus 14.

[0018] An active filter 20 is shunt-connected to the power bus 14,between the ac power source 12 and the loads 16, upstream of anynon-linear loads. The active filter 20 supplies harmonic currents to thenon-linear devices such that the ac power source 12 supplies onlycurrent at the fundamental frequency to the power bus 14. Thefrequencies of the harmonic currents are, for example, (6n±1)f, where fis the fundamental frequency of the ac power generated by the powersource 12 and n is an integer. These harmonic currents are referred toas “characteristic” harmonics. Other harmonics can exist, such as thosecaused by resonant oscillation of filters that are excited by otherconducted emissions injected into the power bus 14. These harmonics arereferred to as non-characteristic harmonics, since they are notintimately related to the fundamental frequency, as are thecharacteristic harmonics. The active filter 20 deals with both thecharacteristic and non-characteristic harmonics.

[0019] Reference is now made to FIG. 2. The active filter 20 includes aninverter 52, a dc link capacitor 54 connected in shunt configurationwith the inverter 52, an inverter control 56, and an output filter 58.The output filter 58 includes a bank of capacitors 60, current sensors62 connected in series with the capacitors 60, inductors 64 and invertercurrent sensors 66.

[0020] The capacitors 60 are connected in shunt configuration to thepower bus 14. The bank of capacitors 60 is required in aircraft systemsto meet power quality and conducted emissions.

[0021] In a typical aircraft system, the current flowing through thecapacitor current sensors 62 is about 15-20 amps. In contrast, thecurrent flowing through the power bus 14 can reach 500 amps or higher.Sensing the smaller currents in the capacitors 60 gives a much betterresolution of the harmonic currents.

[0022] The inverter control 56 controls the inverter 52 to injectharmonic currents into the power bus 14 so as to eliminate the harmoniccurrent in the bank of capacitors 60, and to maintain the dc linkcapacitor 54 at essentially a constant voltage. The inverter 52 may be aconventional six-switch inverter with associated snubbers and diodes, orany other form of voltage controlled inverter.

[0023] The control methodology of the inverter control 56 involves Parkvectors. Park vectors inherently contain information on both theinstantaneous magnitudes and the phase relationship of three phaserotating fields with respect to a reference coordinate system. A Parkvector, in general, is a mathematical representation that describes thelocus of an electrical quantity in the complex space domain (where timeis a parameter). A voltage Park vector is defined with the vector'samplitude and the vector's direction in spatial relation to the threephases. A general discussion of Park vectors is provided in P. K.Kovacs, “Transient Phenomena in Electrical Machines,” Elsevier SciencePublishing Co. (1984).

[0024] Reference is now made to FIG. 3, which shows an exemplary controlmethodology for the inverter control 56. A Park vector representation ofline-to-neutral voltage of the capacitor bank voltage (V_(CAP)) isgenerated from measurements of voltages (Va, Vb, Vc) across each of thecapacitors 60 (block 102) (see FIG. 2). Voltage sensors 68 may takethese measurements.

[0025] The angle (θ) of the capacitor bank voltage Park Vector (V_(CAP))(also referred to as the angle of the fundamental) is estimated fromthis vector (V_(CAP)) by an estimator 104. The estimator 104 performs avector cross-product multiplication (block 106) of the capacitor voltagePark vector and a complex rotator (e^(jθ)) (block 108). The result ofthis vector multiplication is operated upon by a PI regulator 110 whoseoutput (ω) is the estimated angular velocity of the capacitor bankvoltage Park Vector (V_(CAP)). The estimated angular velocity (ω) isintegrated (block 112) to produce the estimated angle (θ) of thecapacitor bank voltage Park Vector (V_(CAP)). This closed loop of theestimator 104 continuously updates the estimate of the angle (θ),thereby reducing the error between the capacitor bank voltage Parkvector (V_(CAP)) and the complex rotator (e^(jθ)).

[0026] The estimated angle (θ) of capacitor bank Park Vector (V_(CAP))is used to derive reference frame rotators of the form e^(sgn(jmθ)),where harmonic number m=(6n±1). The reference frame rotators are used totransform the capacitor current Park vectors to synchronous referenceframes defined by the harmonic number and sequence. If n=1, 2, 3, . . ., nk then a total of 2nk different frame rotators for nk differentcharacteristic harmonics can be computed when their sequence is takeninto account. The individual characteristic harmonics can have apositive sequence (where sgn =−1) or a negative sequence (where sgn=+1).

[0027] The reference frame rotators (e^(sgn(jmθ))) (blocks 115 a and 115b) are supplied to multiple control loops 114. Each control loop 114generates a voltage command (V_(CMD1) to V_(CMDnk)) that results in aharmonic voltage being generated to eliminate the sensed characteristicharmonic currents in the bank of capacitors 60. A separate control loop114 is provided for each harmonic current that is to be eliminated. Forexample, five control loops would be provided to eliminate fivedifferent harmonic currents in the bank of capacitors 60. (Not allharmonics and associated sequences are required to be eliminated in aspecific application; only those harmonics that are desired to beeliminated will be selected for a specific application.)

[0028] Each control loop 114 receives the Park vector (I_(CAP))representing current in the bank of capacitors 60. The current Parkvector (I_(CAP)) may be computed (block 116) from the currents measuredin the capacitor bank by the series-connected set of current sensors 62.

[0029] Within each control loop 114, the current Park vector (I_(CAP))is transformed to a reference frame that is synchronous with respect tothe positional angle mθ (where m is the harmonic number, and theta isthe estimated angle of the fundamental). The transformation to thesynchronous reference frame may be performed by multiplying (block 118)the current Park vector (I_(CAP)) with the complex rotator e^(sgn(jmθ))(block 115 a). The resulting synchronous vector in the m^(th)synchronous reference frame is passed through a low pass filter 120,which filters out all frequencies and leaves a dc value representing thespecific harmonic in its appropriate reference frame. For example, thefifth harmonic (n=1, m=5) viewed in the fifth harmonic reference framewill be represented by a dc value. The filtered signal (in the form of adc voltage) is compared to a zero reference (block 122). The resultingerror, which represents the difference between the desired value of them^(th) harmonic (typically zero), and the computed value in itsappropriate reference frame, is operated upon by a vector PI regulator124, whose output produces the voltage vector required to eliminate thespecific harmonic. This voltage vector is then transformed back (block126) to the stationary frame by multiplying by the complex rotatore^(−sgn(jmθ)) (block 115 b). Resulting is a voltage Park vector (commandV_(CMDm)) representing the m^(th) harmonic voltage required to generatethe m^(th) harmonic current in the inverter 52, which cancels the m^(th)harmonic current in the bank of capacitors 60.

[0030] The voltage Park vectors (V_(CMD1) to V_(CMDnk)) from the controlloops 114 are summed (block 128) with a Park vector (V_(INV))representing inverter fundamental voltage. The sum is used as a voltagecommand (V_(E)) for Space Vector Modulation (SVM) logic 130. The SVMlogic 130 uses space vector modulation to command gate logic 132 to turnon and off the switches of the inverter 52. The switches of the inverter52 may be modulated at a high frequency (e.g., 100 kHz) in order tominimize the size of the capacitors 60 and to enable compensation ofcharacteristic harmonics up to a frequency that is limited by themodulation frequency.

[0031] The inverter control 56 described thus far does not containprotection to the inverter 52 with respect to overcurrent. Therefore anadditional control loop is added to provide this overcurrent protectionwithout introducing additional harmonics.

[0032] Reference is now made to FIG. 4, which shows an exemplary controlmethodology for limiting current to the inverter 52. An overcurrentregulator 202 includes a block 204 for computing a Park vectorrepresenting current flowing through the inverter 52 (using the currentsmeasured by the inverter currents sensors 66) and a peak currentamplitude detector 206 that monitors the magnitude of the invertercurrent Park vector. The peak detector 206 stores the maximum or peakvalue of the magnitude of the inverter current Park vector. At a summingjunction 208 an output of the peak detector 206 is compared to anovercurrent reference (OC), and the resulting error signal is operatedupon by a PI regulator 210. An output of the PI regulator 210 is thenapplied to a plurality of multipliers 212, each of which is connectedaround their respective control loops 114. As the output of the PIregulator 210 increases, the gain of each control loop 114 is reduced,providing droop to each of the control loops 114 and causing the controlloops 114 to reduce the amount of harmonic attenuation, thereby reducingthe inverter current. In this way, a linear reduction in the harmonicattenuation is achieved without generating additional harmonics.

[0033] This single overcurrent regulator 202 can control the droop inall control loops 114 in a parallel or sequential manner so as toprovide overcurrent protection to the inverter 52.

[0034]FIG. 5 shows an overcurrent regulator 202 that operatessequentially on the control loops. The output signal of the overcurrentregulator 202 is supplied in parallel to a series of blocks 250. As theoutput signal from the overcurrent regulator 202 increases from zero,each block 250 increases its output signal by a proportional value,until a limit is reached. When the limit is exceeded, each block 250outputs a maximum value. The outputs of the blocks 250 are progressivelydelayed such that the first block 250 outputs a signal to the multiplier212 associated with the first control loop, then second block 250outputs a signal to the multiplier 212 associated with the secondcontrol loop, then the third block 250 outputs a signal to themultiplier 212 associated with the third control loop, and so on untilthe multiplier 212 associated with the nkth control loop receives asignal. Each multiplier 212 is connected around its associated controlloop.

[0035] Each block 250 has a different initiation level for control, sothat sequential operation is achieved. The advantages of such a schemeare that low order harmonic currents, which usually have the highestamplitude, can be attenuated at the expense of other higher orderharmonics.

[0036] A stable and controlled voltage should be maintained on the dclink capacitor 54. Two different controls 302 and 402 for maintainingthe stable and controlled voltage are shown in FIGS. 6 and 7.

[0037] Referring to FIG. 6, a first control 302 includes a sensor 304for measuring voltage on the dc link capacitor 54 and a summing junction306 for comparing a reference voltage (V_(REF)) to the measured dc linkcapacitor voltage. The resulting error is operated upon by a PIregulator 308, whose output corresponds to the direct-axis (real)component (id) of an inverter vector command (i*). The complete vectorcommand (i*) is constructed by combining a zero quadrature-axis currentcomponent (iq=0) with the direct axis component (block 310).

[0038] An inner current loop 312 regulates the inverter current tomaintain a stable and controlled voltage on the dc link capacitor 54.The inverter current (I_(INV)), represented by the Park vector createdfrom the currents measured by the inverter current sensors 66, istransformed to the fundamental reference frame by multiplying theinverter Park vector (I_(INV)) by e^(−jθ)(block 314).

[0039] This transformed vector is subtracted from the inverter vectorcommand (i*) at a summing junction 316, and the resulting error signalis operated upon by a PI current regulator 318. The output of thecurrent regulator 318 is transformed back to the stationary referenceframe by multiplying current regulator output by e^(+jθ)(block 320). Thetransformed output (V_(INV)) is summed with the voltage commands(V_(CMD1) through V_(CMDnk)) at a summing junction 128 and an output ofthe summing junction 128 is supplied to the SVM logic 130.

[0040] The SVM logic 130 performs space vector modulation by commandingthe gate logic 132 to select inverter switches that create a rotatingvoltage vector. The rotating vector produces a sinusoidal current thatbest matches the commanded inverter current. Typical SVM algorithms maybe used to compute duty cycles and select appropriate voltage vectors orthe null vector so that the time-averaged vector produced approximatesthe commanded voltage vector.

[0041] Two problems can occur in the control 302 of FIG. 6. First, thereexists an undamped filter comprised of the inductance 18 associated withthe distribution system and the source impedances of the loads, and thecapacitors 60 in the capacitor bank of the output filter 58. Second, theinner current loop 312 associated with the DC link voltage regulator 302interferes with the control loops 114.

[0042] Reference is now made to FIG. 7, which shows a control 402 thatcan overcome these two problems. The control 402 includes a damper loop404 and a characteristic current harmonic extractor 406.

[0043] The damper loop 404 provides active damping to the resonance ofthe capacitors 60 of the capacitor bank and external inductance, and thecharacteristic harmonic extractor 406 removes all characteristicharmonics from the inverter current Park vector so that the multiplecontrol loops 114 and the inner current loop 312 do not interfere withone another. In this manner, interaction between the multiple controlloops is eliminated.

[0044] The damper loop 404 takes the capacitor bank voltage Park vector(V_(CAP)), and transforms this vector to the fundamental frequencyreference frame (block 408). The resulting signal is passed through ahigh pass filter 410, which passes the high frequency signalsrepresenting the resonant voltage superimposed upon the fundamental.This is compared to a zero reference signal at summing junction 412, andthe resultant signal operated upon by a damper regulator 414. The damperregulator 414 attenuates any resonance that could be caused byextraneous excitation exciting the output capacitor. The output of thedamper regulator 414 is summed with the current Park vector command atthe summing junction 316. The damper loop 404 does not impact the gainof the inner current loop 312; its function is damp outnon-characteristic oscillations on the capacitors 60 of the capacitorbank.

[0045]FIG. 8 shows the characteristic harmonic extractor 406 in greaterdetail. For each harmonic that is regulated by the control loops 114,there is a corresponding block 502 in the harmonic extractor 406. Thusan m^(th) harmonic extractor block 502 will be provided for the m^(th)harmonic. Each harmonic extractor block 502 performs a co-ordinatetransformation upon the inverter current Park vector (I_(INV)) withrespect to the appropriate harmonic frequency and sequence (block 504),low-pass filters the resulting signal (block 506), converts the filteredsignal back to the stationary reference frame (block 508), and subtractsthe m^(th) harmonic current Park vector from the sensed inverter currentPark vector (I_(INV)). The transformation and low pass filteringeffectively allow the removal of the harmonic currents from the invertercurrent Park vector (I_(INV)), thereby eliminating the possibility ofinteraction between the multiple loops.

[0046] Thus disclosed is a method and apparatus for dealing withharmonic currents in variable frequency distribution systems. The methodand apparatus allow a main power source to provide ac power having apurely sinusoidal waveshape.

[0047] Because currents are sensed in the capacitor banks instead of thepower line, the sensing of lower currents allows for greater resolutionof the harmonic currents. It also allows smaller current sensors to beused. Moreover, the harmonic currents are dealt with without cuttinginto the main distribution system.

[0048] Current limiting eliminates harmonic currents that exceed therating of the inverter. Moreover, inverter current-limiting can beperformed without introducing additional harmonics due to theimplementation of current limit function.

[0049] Parallel operation of multiple active filters does not requireany additional circuitry, or information transfer between the inverters.The sensing of current in the capacitor bank ensures balance between allinverters; unbalance is determined solely by the tolerance of the outputcapacitors.

[0050] The selection of harmonics to be eliminated is programmable, asis their sequence.

[0051] Active damping is provided. Bulk filter, control loop bandwidthand modulation frequency are all optimized.

[0052] Although the system is described in connection with three-phaseac power, it is not so limited. The system may instead utilize two-phaseac power.

[0053] Although the power distribution system has been described as avariable frequency system, it is not so limited. The power distributionsystem may be a constant frequency system. The method and apparatus maybe applied to any frequency system (including dc systems).

[0054] Although the power distribution system has been described inconnection with an aircraft distribution system, it is not so limited.The power distribution system may be used in military, space andindustrial applications.

[0055] The active filters may be used to remove harmonic currents from avoltage source including a single generator or multiple generatorsoperating in parallel.

[0056] The PI regulators of the active filters may be scalar regulatorswhen appropriate synchronous reference frames are used. In thealternative, complex regulators in the stationary reference frame may beused. A single complex regulator would be used in a stationary frame foreach harmonic current.

[0057] The inverter control may be implemented in hardware, software orany combination of the two. For example, the inverter control may beimplemented as a digital signal processor.

[0058] The power distribution system is not limited to SVM logic forturning on and off the switches of the inverter. Other vector modulationschemes may be used.

[0059] The present invention is not limited to the specific embodimentsdescribed above. Instead, the present invention is construed accordingto the claims that follow.

What is claimed is:
 1. A power distribution system comprising: an ac power source; a power bus connected to the ac power source; a capacitor bank shunt-connected to the power bus; and an active filter shunt-connected to the power bus, the active filter including an inverter, an inverter control and current sensors, each current sensor sensing current flowing through a corresponding capacitor of the capacitor bank, the inverter control, in response to the current sensors, controlling the inverter to inject harmonic currents into the power bus.
 2. The system of claim 1, wherein the inverter control controls the inverter to inject harmonic currents so as to eliminate harmonic currents in the capacitor bank.
 3. The system of claim 1, wherein the harmonic currents are characteristic harmonic currents.
 4. The system of claim 1, wherein the active filter supplies harmonic currents such that the ac power source supplies current to the power bus at a fundamental frequency only.
 5. The system of claim 1, wherein the active filter includes a plurality of control loops, each control loop causing the filter to inject a different harmonic current into the power bus.
 6. The system of claim 5, wherein each control loop generates a voltage command corresponding to a characteristic harmonic, and wherein the active filter further includes means for summing the voltage command with a command corresponding to inverter voltage, and vector modulation logic, responsive to an output of the summing means, for controlling the inverter.
 7. The system of claim 6, wherein the active filter derives reference frame rotators of the form e^(sgn(jmθ)), where m=(6n±1), different rotators being used by different control loops.
 8. The system of claim 6, wherein each loop is responsive to a positive or negative sequence angle of the Park vector of capacitor bank current.
 9. The system of claim 6, wherein within each control loop, the current Park vector is transformed to a reference frame that is synchronous with respect to positional angle (6n±1)θ of the fundamental; the resulting synchronous vector in the m^(th) synchronous reference frame is passed through a low pass filter, which filters out all frequencies; and the filtered signal is supplied to a PI regulator, which provides the voltage command for generating the corresponding harmonic current.
 10. The system of claim 5, wherein the active filter further includes an inverter overcurrent regulator.
 11. The system of claim 10, wherein the overcurrent regulator changes gain of the control loops in inverse proportion to an increase in overcurrent.
 12. The system of claim 11, wherein overcurrent regulator operates on the control loops in parallel.
 13. The system of claim 11, wherein overcurrent regulator operates on the control loops sequentially.
 14. The system of claim 1, further comprising a dc link capacitor coupled across the inverter, wherein the inverter control also controls the inverter to maintain the dc link capacitor at essentially a constant and stable voltage.
 15. The system of claim 14, wherein the inverter control generates an inverter vector command having a zero quadrature-axis component and a direct-axis component derived from dc link capacitor voltage; and includes an inner loop for regulating measured inverter current according to the inverter vector command.
 16. The system of claim 15, wherein the inverter control includes a damping loop for modifying the inner loop to damp out non-characteristic oscillations on the capacitor bank.
 17. The system of claim 16, wherein the inverter control further removes harmonic currents from a Park vector representing the measured inverter current.
 18. An active filter for a power distribution system, the system including a power bus, the filter comprising: an inverter; means for generating a plurality of different voltage commands, each voltage command corresponding to a different harmonic current; means for summing the different voltage commands with a voltage command representing inverter voltage; and means, responsive to the summing means, for controlling the inverter to inject harmonic currents into the power bus.
 19. An active filter for a power distribution system, the system including a power bus and a capacitor bank shunt-connected to the power bus, the filter comprising: an inverter; and a plurality of control loops, each control loop corresponding to a different multiple of capacitor bank Park Vector angle, each control loop causing the inverter to inject a different harmonic current into the power bus.
 20. The active filter of claim 19, wherein each control loop generates a voltage command corresponding to a characteristic harmonic, and wherein the active filter further comprises means for summing the voltage command with a command corresponding to inverter voltage, and vector modulation logic, responsive to an output of the summing means, for controlling the inverter.
 21. The active filter of claim 19, wherein within each control loop, a Park vector representing capacitor bank current is transformed to a reference frame that is synchronous with respect to positional angle (6n±1)θ of the fundamental; the resulting synchronous vector in the m^(th) synchronous reference frame is passed through a low pass filter, which filters out all frequencies; and the filtered signal is supplied to a PI regulator, which provides the voltage command for generating the corresponding harmonic current.
 22. The filter of claim 19, further comprising an inverter overcurrent regulator for changing gain of the control loops in inverse proportion to an increase in overcurrent.
 23. The filter of claim 19, further comprising a dc link capacitor coupled across the inverter, the dc link capacitor providing power to the inverter; and an inverter control for controlling the inverter to maintain the dc link capacitor at essentially a constant and stable voltage.
 24. The active filter of claim 19, further comprising an inverter control for generating an inverter vector command having a zero quadrature-axis component and a direct-axis component derived from dc link capacitor voltage; and an inner loop for regulating measured inverter current according to the inverter vector command.
 25. The active filter of claim 24, wherein the inverter control includes a damping loop for modifying the inner loop to damp out non characteristic oscillations on the capacitor bank; and logic for removing harmonic currents from the measured inverter current Park vector.
 26. A method of using an inverter to filter harmonic currents on a power bus of a power distribution system, a capacitor bank being shunt-connected across the power bus, the method comprising: measuring currents flowing through the capacitors of the capacitor bank; and controlling the inverter to inject harmonic currents into the power bus in response to the measured currents so that the inverter supplies harmonic current demands of non linear loads on the power bus.
 27. The method of claim 26, wherein voltage commands corresponding to multiple characteristic harmonics are generated, and wherein the voltage commands are summed with a command corresponding to inverter voltage, and wherein the inverter is vector-modulated in response to the sum. 